Liquid crystal display elements are used in not only watches and electronic calculators, but also various measuring instruments, panels for automobiles, word processors, electronic notebooks, printers, computers, televisions, clocks, advertisement display boards, etc. Typical examples of a liquid crystal display system include a twisted nematic (TN)-type display, a super-twisted nematic (STN)-type display, a vertical alignment (VA)-type display using a thin-film transistor (TFT), and an in-plane-switching (IPS)-type display. It is desired that liquid crystal compositions used in these liquid crystal display elements be stable against external factors such as moisture, air, heat, and light, exhibit a liquid crystal phase in as wide a temperature range as possible with room temperature being at the center of the range, have low viscosities, and have low drive voltages. Furthermore, the liquid crystal compositions each contain several types to several tens of types of compounds for the purpose of optimizing, for example, dielectric anisotropy (Δ∈) and/or birefringence (Δn) for respective display elements.
In vertical alignment displays, liquid crystal compositions having a negative Δ∈ are used, and such displays are widely used in liquid-crystal display televisions and the like. In all driving systems, low-voltage driving, high-speed response, and a wide operating temperature range have been desired. Specifically, a large absolute value of Δ∈, a low viscosity (η), and a high nematic phase-isotropic liquid phase transition temperature (Tni) have been desired. Furthermore, it is necessary to control the Δn of a liquid crystal composition to be in an appropriate range in accordance with a cell gap on the basis of a value of Δn×d, which is the product of the Δn and the cell gap (d). In addition, in the case where a liquid crystal display element is applied to a television or the like, high-speed responsiveness is important and thus a liquid crystal composition having a small rotational viscosity γ1 is required. In particular, recently, in order to reduce the cell gap for realizing high-speed response, it has been necessary to increase the Δn in addition to the decrease in the viscosity. For this purpose, PTL 1 and PTL 2 disclose a liquid crystal composition containing a compound having a terphenyl structure substituted with a fluorine atom.
In order to practically use a liquid crystal composition in a liquid crystal display element, it is necessary that no problem occur in terms of display quality. In particular, a liquid crystal composition used in an active-matrix driving liquid crystal display element that is driven by TFT elements or the like needs to have a high specific resistance or a high voltage holding ratio. In addition, it is also necessary for such a liquid crystal composition to be stable against external stimuli such as light and heat. To meet this requirement, antioxidants for improving stability against heat and liquid crystal compositions containing the antioxidants have been disclosed (refer to PTL 3 and PTL 4). However, the stability is not necessarily sufficient. In particular, liquid crystal compounds having a large Δn have relatively low stability against light and heat, and thus such a composition does not have sufficient quality stability.
Furthermore, with the increasing number of applications for liquid crystal display elements, methods of using the liquid crystal display elements and methods of producing the liquid crystal display elements have also been significantly changed. In order to catch up with these changes, it has been desired to optimize properties other than known basic physical property values. Specifically, regarding liquid crystal display elements that use liquid crystal compositions, vertical alignment (VA)-type liquid crystal display elements, in-plane-switching (IPS)-type liquid crystal display elements, and the like have been widely used, and very large display elements having a 50-inch or larger display size have been practically used. Regarding a method for injecting a liquid crystal composition into a substrate, with the increase in the size of substrates, a one-drop-fill (ODF) method has been mainly used instead of an existing vacuum injection method (refer to PTL 5). However, it has been found that a drop mark formed when a liquid crystal composition is dropped on a substrate results in a problem of a decrease in the display quality. In order to form a pre-tilt angle of a liquid crystal material in a liquid crystal display element and to realize high-speed responsiveness, polymer-stabilized (PS) liquid crystal display elements and polymer-sustained alignment (PSA) liquid crystal display elements have been developed (refer to PTL 6). However, the problem of a drop mark has become a significant problem. These display elements are characterized in that a monomer is added to a liquid crystal composition and that the monomer in the composition is cured. In many cases, the monomer is cured by irradiating the composition with ultraviolet light. Therefore, in the case where a component having low stability against light is added to the liquid crystal composition, the specific resistance or the voltage holding ratio is decreased, and in some cases, the generation of a drop mark may also be induced, resulting in a problem of a decrease in the yield of the liquid crystal display element due to display defects.
Accordingly, it has been desired to develop a liquid crystal display element which has high stability against light, heat, etc. and in which display defects such as image sticking and a drop mark do not tend to occur while maintaining characteristics and performance, such as high-speed responsiveness, which are desired for the liquid crystal display element.